General-Purpose Computer and Communication Interface

The five GPCs in the orbiter are loaded with software (to perform different functions) during different phases of the mission. During ascent and entry, four of the GPCs are loaded with primary avionics software system to provide guidance, navigation and control. One computer is loaded with backup flight system software and can take control of the orbiter if the PASS GN&C; computers malfunction. The BFS GPC also contains all software necessary to return the orbiter to Earth, if required. Normal communication commands go through the BFS during ascent and entry phases of the mission. During the on-orbit phase of the mission, one computer is loaded with systems management software to support the communication commands. These two GPCs process antenna-pointing control commands for the S-band and Ku-band antenna systems. They also provide a communication command path from the ground to the required subsystem as well as a telemetry downlist path to the PCMMU to be interleaved with other data for transmission to the ground.